The present invention relates to drivers, and in particular, to systems and methods of driving an electroluminescent lamp.
Electroluminescent lamps are known as very low efficiency lamps. For example, a good electroluminescent lamp will yield about three to five lm/W of input power while a good fluorescent lamp will yield about 50 to 80 lm/W. The advantage they offer over traditional lighting though is their ability to provide even illumination over a flat area. Because of this, they are the preferred lamp in relatively low-intensity situations where it is important to obtain even glow over an entire area. Example applications may include providing backlight for LCDs in pagers, cell phones, watches, and control systems. Additionally, some lamps such as LEDs may require the use of a light guide to achieve light uniformity. However, this may increase the form factor. Many applications of EL lamps do not require a light guide, which makes them a good choice for applications requiring a thin form factor.
Since many of the applications for electroluminescent lamps are portable, circuits driving the lamp typically take a DC source, such as a battery, and transform the voltage into an AC signal at approximately 100V and 100 Hz. The brightness of the electroluminescent lamp is proportional to the applied voltage so it is desirable to reach the voltage required for normal operation as quickly as possible. FIG. 1 illustrates typical signal driving an electroluminescent lamp. The output voltage is received by an electroluminescent lamp from a driver circuit, such as an H-bridge type driver. In the first half-cycle, the output voltage transitions to the maximum voltage for normal operation and held there until the end of the half-cycle. Similarly, the output voltage transitions to the minimum voltage for normal operation and held there until the end of the half cycle. One problem that surfaces when driving an electroluminescent lamp in this manner is the occurrence of audible noise. The rapid change in high voltage may cause the thin sheets of material used in the creation of electroluminescent lamps to change shape and create an audible noise. This is known as the piezoelectric effect. If the voltage is raised too quickly, an audible noise may be generated by the lamp due to the piezoelectric effect. Conversely, if the voltage is raised too slowly in an attempt to avoid the piezoelectric effect, the lamp may not be able to maintain the level of illumination.
Thus, there is a need for an improved system and method of driving an electroluminescent lamp. The present invention solves these and other problems by providing a system and method of noise reduction for electroluminescent lamp drivers.